This invention relates to multi-section telescopic booms for truck mounted cranes or the like, and particularly to a unique boom construction which has a substantially greater load capacity for a given boom weight.
Most current telescoping booms have solid or essentially solid side, top and bottom walls, and the boom of this invention is intended primarily to overcome problems that would arise in providing greater load capacity using this construction. Full lattice booms are also known and may avoid some of the problems discussed below; but it is difficult for various reasons to construct large lattice booms, one problem being the provision of suitable bearing surfaces for relatively telescoping sections.
In designing a boom to lift a heavier load, the height of the side walls must obviously be increased to provide the needed vertical rigidity; however, the width of the boom sections must also be increased to prevent excessive twisting or lateral flexing of the boom about its longitudinal axis. Booms with a solid wall construction cannot as a practical matter be scaled up for very high loads, since a limit is ultimately reached beyond which the increase in size results in an inordinate increase in weight, and a net decrease in lifting capacity.
Another problem with conventional solid wall constructions arises from the fact that inner wear pads are usually mounted on an inner section and ride along the top wall of the next outer section and these pads exert tremendous upward forces on the top wall of the outer section. Consequently, the thickness of the top wall must usually exceed that of the other walls to withstand these forces, and this thickness must be uniform throughout the boom length so that a continuous bearing surface is present to accommodate any desired relative position between the sections. In a very large capacity boom, the increase in weight resulting from the material which must be added to the top walls of the appropriate sections increases the overall weight of the boom beyond acceptable limits.
Another problem which occurs as the cross section size of a solid wall boom is increased is a corresponding inefficient consumption of material. That is, some portions of the walls are not subjected to the same forces as others, but it would not be practical to vary wall thickness and so the whole wall is made of maximum thickness, resulting in both excess weight and waste of material.